linux-kernel - Re: [PATCH] mm/hugetlb: avoid corrupting page->mapping in hugetlb_mcopy_atomic

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Message-ID: <Ys9XAbuxkDUlnmv0@monkey>
Date:   Wed, 13 Jul 2022 16:36:33 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Axel Rasmussen <axelrasmussen@...gle.com>
Cc:     Peter Xu <peterx@...hat.com>, Miaohe Lin <linmiaohe@...wei.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Muchun Song <songmuchun@...edance.com>,
        Linux MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm/hugetlb: avoid corrupting page->mapping in
 hugetlb_mcopy_atomic_pte

On 07/13/22 15:46, Axel Rasmussen wrote:
> I think there is a small mistake in this patch.
> 
> Consider the non-minor-fault case. We have this block:
> 
> /* Add shared, newly allocated pages to the page cache. */
> if (vm_shared && !is_continue) {
>         /* ... */
> }
> 
> In here, we've added the newly allocated page to the page cache, and
> we've set this page_in_pagecache flag to true. But we *do not* setup
> rmap for this page in this block. I think in this case, the patch will
> cause us to do the wrong thing: we should hugepage_add_new_anon_rmap()
> further down, but with this patch we dup instead.

I am not sure I follow.  The patch from Miaohe Lin would not change any
behavior in the 'if (vm_shared && !is_continue)' case.  In this case
both vm_shared and page_in_pagecache are true.

IIUC, the patch would address the case where !vm_shared && is_continue.

On 07/12/22 21:05, Miaohe Lin wrote:
> In MCOPY_ATOMIC_CONTINUE case with a non-shared VMA, pages in the page
> cache are installed in the ptes. But hugepage_add_new_anon_rmap is called
> for them mistakenly because they're not vm_shared. This will corrupt the
> page->mapping used by page cache code.
> 
> Fixes: f619147104c8 ("userfaultfd: add UFFDIO_CONTINUE ioctl")
> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
> ---
>  mm/hugetlb.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 8d379e03f672..b232e1508e49 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -6038,7 +6038,7 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
>  	if (!huge_pte_none_mostly(huge_ptep_get(dst_pte)))
>  		goto out_release_unlock;
>  
> -	if (vm_shared) {
> +	if (page_in_pagecache) {
>  		page_dup_file_rmap(page, true);
>  	} else {
>  		ClearHPageRestoreReserve(page);

-- 
Mike Kravetz